This invention relates to the field of the electrolytic deposition of metals and, more particularly, to an improved method of evaluating electrolytic systems with respect to the detection of impurities in the electrolytic solution of such systems, the current efficiency characteristics of the solution and the characteristics of electrodes and electrode materials of the systems.
The electrolytic deposition of metals finds extensive commercial use in a variety of applications. Certain metals such as zinc and copper are often recovered by electrowinning techniques in which a solution obtained by leaching an ore concentrate with sulfuric acid is subjected to electrolysis, with resultant deposition of the metal at the electrolytic cathode. Additionally, metals which have been recovered in crude form from ores by pyrometallurgical techniques are in some instances refined electrolytically. Thus, copper produced by conventional smelting and converter processes is purified by dissolving it in sulfuric acid and electrolytically depositing it on a cathode with attendant concentration of nickel and noble metals in the electrolytic solution or a precipitated sludge. The electrodeposition of the metals finds further extensive use in the application of protective or decorative metal platings such as chromium, nickel, silver and gold.
In certain electrodeposition processes, very minor amounts of impurities can have a major effect on the operation of the process and the quality of the product. In the electrowinning of zinc, for example, the current efficiency is sensitive to antimony in the parts per billion range since very small amounts of antimony apparently materially lower the hydrogen overvoltage, thus diverting a considerable portion of the electrolytic current to the liberation of hydrogen rather than the deposition of zinc. Relatively larger, but nonetheless very minor, amounts of more noble metals such as silver, gold, copper, platinum, cobalt and nickel also lower hydrogen overvoltage by plating out on the cathode. Antimony, together with germanium, selenium or tellurium, may have the further adverse effect of promoting the redissolution of zinc with a consequent catastrophic effect on current efficiency.
Although antimony is commonly present as an impurity in zinc electrowinning solutions and has a highly adverse effect on current efficiency, the presence of very small amounts of antimony has the favorable effect of inhibiting the nonreleasable sticking of zinc to the aluminum cathodes conventionally used in zinc electrowinning. Moreover, the catastrophic adverse effect on current efficiency caused by excess antimony can be largely offset by the presence of a protein glue additive. In a similar fashion, minor proportions of 1-nitroso-2-naphthol tie up and offset the otherwise adverse effects of nickel and cobalt. Very minor amounts of a protein glue also promote a favorable zinc deposit morphology and prevent dendrite growth which could otherwise lead to cell shorting.
In the electrodeposition of copper, relatively minor amounts of chloride ion normally have a polarizing effect, reducing current densities at given voltages. Other additives such as glue, thiourea and anionic flocculating agents may also have significant effects on deposit morphology and/or other important aspects of the electrodeposition process. Control of morphology is a major problem in copper electrodeposition processes.
Because of the strong influence of very minor proportions of impurities and additives, normal quantitative analytical techniques are not effective in predicting the performance characteristics of an electrolytic solution used in an electrodeposition process. For this reason, such techniques are often inadequate to provide definitive guidance for the treatment necessary to improve the performance of a solution already charged to the cells. Moreover, such analyses as can be made frequently do not directly or readily correlate with performance characteristics. Under current practice in zinc electrowinning, for example, additions of additives such as glue may be made in response to plant observations but the number of variables which affect the ultimate plant response, together with the significant impact of very minor proportions of glue, result in the process control procedure being a somewhat hit-and-miss, trial-and-error proposition.
Further problems may be encountered in electrodeposition processes because of imperfections in the electrodes. In zinc electrowinning, for example, the presence of an oxide coating on the aluminum cathodes is essential to prevent sticking of the deposited zinc to the cathodes. Prior to the present invention, however, there have been few, if any, highly reliable techniques for predicting the performance of a cathode before it is installed in a commercial cell.
If more definitive process measurement and control techniques were made available, significant improvements could be made in the economy and reliability of various electrodeposition processes. Thus, for example, if a technique were available by which the current efficiency obtainable from a particular electrolytic solution could be quickly and reliably predicted before that solution is delivered to the electrolytic cell, the average current efficiencies achieved could be materially increased. If a quantitative determination could be made of the minor amounts of impurities which significantly affect current efficiency, a further major advantage could be realized both with respect to control of the electrolytic solution delivered to the cells and with respect to the control of additives by which cell liquor composition can be modified. A still further major advantage would arise from the capability of preventing dendrite growth or other morphological defects rather than attempting to correct these tendencies once they have been observed in the cell.